Stutter
The new lad at work told us on his first day, that he had a stammer which he usually controlled, but sometimes came out when he was nervous. He asked us just to be patient if he did, which of course we all agreed. It happened a couple of times, and everyone just politely waited for him to get the next word out.

And then in walks Geoff. An older chap. Yorkshireman. A bit (as is their wont) forthright in manner.

"Awright lad, how're you finding it with these twats?""Umm... D-...D-..D-....""Come on, spit it out!!"